The present invention relates to synchronous detector systems and, more particularly, to a television intermediate frequency (IF) video demodulator system.
Most television IF demodulator systems in use today are of one of two types. A first type is commonly referred to as a pseudo-synchronous video detector. An example of this type of detector is the MC1330 Low Level Video Detector circuit manufactured by Motorola, Inc. which is described in U.S. Pat. No. 3,697,685. In this system the video information is recovered by first limiting the IF video carrier signal then filtering and applying it to the first inputs of a balanced multiplier. The video information is then recovered by applying the original video carrier signal to second inputs of the multiplier as is well known.
In the second common type of video detector the same principle is used as described above except the carrier is recovered by phase locking a voltage controlled oscillator (VCO) to the IF signal and then using the VCO output as the video carrier.
The pseudo-synchronous approach suffers from several disadvantages the chief of which is the need for use of a low Q tuned circuit. This tuned circuit gives rise to quadrature distortion which leads to high sound-chroma beat frequency products which is undesirable. In addition the system requires an external tuned circuit to be connected directly to the output of the IF amplifier which causes stability and circuit layout problems. Finally the psuedo-synchronous approach exhibits relatively poor differential gain and phase performance.
The second approach described above has less quadrature distortion due to the narrow phase locked loop (PLL) bandwidth and therefor produces lower beat frequency products. However, to achieve acceptable differential phase and gain performance an extra tuned circuit has to be used at the output of the limiter stage. This causes increased stability problems associated with the IF amplifier stage as well as circuit layout problems. Further, the PLL demodulator system tends to lock to itself due to the VCO oscillating signal, which is at the same frequency as the video carrier, radiating into the input of the IF amplifier which decreases the sensitivity of the demodulator system.
Hence, a need exists for an improved video detector system which overcomes the problems of the prior art systems.